Two-dimensional structure of volume recombination in JT-60U detached divertor plasmas

The volume recombination in detached divertor plasmas is a key process to reduce the ion flux to the divertor plates. Two-dimensional measurement is one of the ways to investigate a spatial structure of the volume recombination. In this work, the deuterium Balmer-series lines (D$_{\alpha }$, D$_{\beta }$, ..., D$_{\theta })$ from a detached divertor plasma were observed two-dimensionally with a spatial resolution of $\sim $ 1 cm and were reconstructed into two-dimensional emissivities with a tomography technique. The ratio of the D$_{\beta }$ to the D$_{\alpha }$ emissivity was compared to that calculated by the collisional-radiative model. This ratio could not be explained only by the excitation of D by electron impact, indicating that the volume recombination contributed to the D$_{\beta }$ emission. This is the case for the region above the inner strike point with $\sim $ 8 cm and $\sim $ 4 cm, respectively, in the r- and the z-direction on the poloidal cross-section. In this region, from the ratios of the D$_{\alpha }$, D$_{\beta }$, ..., D$_{\theta }$ emissivities, the electron density and temperature were evaluated to be $\sim $ 1E20 m$^{-3}$ and $<$ 0.3 eV, respectively.